Amounts of discards by commercial fisheries and their significance as food for seabirds in the North Sea

Abstract

Most types of fishery produce discards and offal in considerable quantities which are fed upon by seabirds. This paper demonstrates the importance to seabirds of fishery waste in the North Sea. The total amount of fishery waste in the North Sea region is estimated at 62800 t of offal, 262200 t of roundfish, 299300 t of flatfish, 15000 t of elasmobranchs and 149700 t of benthic invertebrates per year, representing 4 % of the total biomass of fish and 22% of the total landings This equals an energy value of about 3.4 X 10" kJ. Beam trawl f~sheries discharge discards at the highest rates of all fishing fleets. Their discard fraction is dominated by flatfish which are less favoured by seabirds because of their shape. In contrast, the amounts of discards from pelagic and gadid fisheries are less, but fish species and lengths are more appropriate as food for seabirds. The number of seabirds potentially supported by fishery waste in the North Sea is estimated to be roughly 5.9 million individuals in an average scavenger community (composition in proportion to the seasonal abundance of scavenging species). During expenmental discard studies, the proportions of fishery waste consumed by seabirds was calculated. We estimated that the mass of discards and offal consumed by birds during our study amounted to 55000 t of offal, 206000 t of roundfish, 38000 t of flatfish, 2000 t of elasmobranchs and 9000 t of benthic invertebrates.

Impacts of fisheries on seabird communities

“…Although it is clear from recent research that discards and offal can be a very important food for certain seabirds in many different parts of the world (Abrams, 1983;Dändliker and Mülhauser, 1988;Hudson andFurness, 1988, 1989;Ryan and Moloney, 1988;Blaber and Wassenberg, 1989;Berghahn and Rosner, 1992;Furness et al, 1992;Thompson, 1992;Camphuysen, 1994;Evans et al, 1994;Garthe and Hüppop, 1994;Walter andBecker, 1994, 1997;Blaber et al, 1995;Camphuysen et al, 1995;Thompson and Riddy, 1995;Arcos and Oro, 1996;Garthe et al, 1996Garthe et al, , 1999Chapdelaine and Rail, 1997;Freeman, 1997;Walter, 1997;Walter and Becker, 1998;Freeman and Smith, 1998;Votier et al, 2001), the extent to which this supplementary food supply affects breeding success and population trends of scavenging seabirds is less clear (Howes and Montevecchi, 1992;Noordhuis and Spaans, 1992;Oro, 1996;Oro et al, 1995Oro et al, , 1996aFurness, 1999b;Tasker et al, 1999). There is evidence to indicate large increases of scavenging seabird populations where large quantities of discards have been generated (Furness, 1999b;Chapdelaine and Rail, 1997;Garthe et al, 1999), and evidence suggesting that ...…”

“…A second kind of impact arises where fisheries make available to scavenging seabirds food that they could not naturally obtain for themselves. For example, fisheries catching benthic fish that are too deep for most seabirds to reach, and too large for those able to dive to the seabed to swallow, make these fish available to scavenging seabirds in the form of discards and offal (Hudson, 1989;Hudson andFurness, 1988, 1989;Camphuysen et al, 1995;Garthe et al, 1996;Moore and Jennings, 2000;Ojowski et al, 2001). The quantities of fish discarded by fisheries are enormous.…”

Population dynamics in lesser black-backed gulls in the Netherlands reveal no response to North Sea regime shift: comment on Luczaket al. 2012

“…All common North Sea crab species were consumed (Liocarcinus holsatus, Liocarcinus navigator, Liocarcinus pusillus, Necora puber), but warm water species such as Polybius henslowii, indicative for the reported regime shift, were not encountered. The relative contribution of crabs to gull diets in mass or energetic value [11] was low throughout.…”

Activity patterns and foraging behaviour of Audouin' s gulls in the Ebro Delta, NW Mediterranean

“…Many seabird species obtain large amounts of food as discards from fishing vessels (e.g. Furness et al, 1992;Thompson and Riddy, 1995;Garthe et al, 1996;Walter and Becker, 1997), a foraging resource that is much easier to obtain than the food obtained through "natural" feeding techniques. In the western Mediterranean, this resource is also very predictable both in space and time, especially for trawlers, which always operate in the same fishing grounds off the harbours and in the same timetables (Martín, 1989;Oro, 1995a;Martínez-Abraín et al, 2002).…”